The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical x-ray tubes are built with a rotating anode structure that is rotated by an induction motor comprising a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The x-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces x-rays upon impact with the anode target. The target typically comprises a disk made of a refractory metal such as tungsten, molybdenum or alloys thereof, and the x-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. High speed rotating anodes can reach 9,000 to 11,000 RPM.
Only a small surface area of the target is bombarded with electrons. This small surface area is referred to as the focal spot, and forms a source of x-rays. Thermal management is critical in a successful target anode, since over 99 percent of the energy delivered to the target anode is dissipated as heat, while significantly less than 1 percent of the delivered energy is converted to x-rays. Given the relatively large amounts of energy which are typically conducted into the target anode, it is understandable that the target anode must be able to efficiently dissipate heat. The high levels of instantaneous power delivered to the target, combined with the small size of the focal spot, has led designers of x-ray tubes to cause the target anode to rotate, thereby distributing the thermal flux throughout a larger region of the target anode.
When considering the performance of x-ray tubes, some of the issues of importance are x-ray generation efficiency, patient dose management, high voltage stability, selective spectral content, detector response time and speed of image acquisition.
Present x-ray tube design has an efficiency of around 1 percent, with the remaining power input being dissipated as heat. Large tube targets and accompanying structures are necessary to accommodate this power. Presently, the x-ray tube is powered by two sources, one for heating the filament and the other for supplying the high voltage (HV) accelerating potential across the anode-to-cathode gap. These power sources, whether AC or DC, provide a constant power to the tube resulting in a constant output. This method results in power being dissipated during times when there are no x-rays being generated, or during times when the generated x-rays are not needed or utilized.
It is recognized that using a source of high voltage in a pulsed or resonant method will increase the overall efficiency of the x-ray tube. When the accelerating voltage is generated using a pulsed high voltage supply, the dielectric strength of the insulation system is dependent on the duration of the voltage pulse, i.e. insulators have a higher dielectric strength for short duration pulses. This effect is well-known and reflected in corresponding Voltage-Time Characteristic Curves. These curves apply to most dielectric materials and indicate a voltage that the material can withstand, the breakdown voltage, VBD, that is not constant with respect to the time duration of the application of the high voltage. The Voltage-Time Characteristic Curves reflect that for the same geometry or dielectric spacing, a higher voltage can be applied over short periods of time. Alternatively, the curves reflect that for a given voltage level the spacing or thickness of the dielectric material can be reduced. Thus, in general, the use of pulsed power technology enables the use of smaller HV critical components as compared to DC high voltage application.
The power source for the filament needs to be a more constant source due to the slow thermal response time of the filament structure. This results in a low efficiency application of power and the attendant use of large wires to handle the filament current.
The overall size of the tube is generally a result of the maximum power required. In cases where small focal spots are more important than power, the size of the tube can be made smaller, but is limited by the size of the HV cables. This limits the tube to being hard mounted in a fixture, limiting its usefulness in accessing difficult areas of the anatomy.
Thus, a method and apparatus is desired to eliminate unnecessary electron generation when the electrons are not needed or have a minimum effect on image quality based on the detector response time or the speed of image acquisition. Furthermore, it is desired to reduce the power requirements and thus the cabling size to an x-ray tube and high voltage components therein necessary for electron generation.